[ubsan] Relax nullability-return for blocks with deduced types

When the return type of an ObjC-style block literals is deduced, pick
the candidate type with the strictest nullability annotation applicable
to every other candidate.

This suppresses a UBSan false-positive in situations where a too-strict
nullability would be deduced, despite the fact that the returned value
would be implicitly cast to _Nullable.

rdar://41317163

llvm-svn: 335572

1 files changed, 8 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index b6ea484..98b428b 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -707,8 +707,15 @@ void Sema::deduceClosureReturnType(CapturingScopeInfo &CSI) {
     QualType ReturnType =
         (RetE ? RetE->getType() : Context.VoidTy).getUnqualifiedType();
     if (Context.getCanonicalFunctionResultType(ReturnType) ==
-          Context.getCanonicalFunctionResultType(CSI.ReturnType))
+          Context.getCanonicalFunctionResultType(CSI.ReturnType)) {
+      // Use the return type with the strictest possible nullability annotation.
+      auto RetTyNullability = ReturnType->getNullability(Ctx);
+      auto BlockNullability = CSI.ReturnType->getNullability(Ctx);
+      if (BlockNullability &&
+          (!RetTyNullability || *RetTyNullability < *BlockNullability))
+        CSI.ReturnType = ReturnType;
       continue;
+    }
 
     // FIXME: This is a poor diagnostic for ReturnStmts without expressions.
     // TODO: It's possible that the *first* return is the divergent one.